Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and embedded servo sectors. The embedded servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo controller to control the velocity of the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 6 defined by servo sectors 40-4N recorded around the circumference of each servo track. Each servo sector 4i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 4i further comprises groups of servo bursts 14 (A, B, C, D in the example shown), which are recorded with precise intervals and offsets relative to the track centerlines. The servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations.
In some cases, the servo tracks defined by the servo sectors will comprise an eccentricity due, for example, to a non-centric alignment of the disk with the spindle motor. The eccentricity of servo tracks is particularly apparent when the servo sectors are written by a media writer prior to clamping the disk to the spindle motor of a disk drive as compared to writing the servo sectors after installing the disk into the disk drive. In certain designs, the eccentricity of the servo tracks is accounted for by cancelling the effect from the position error signal (PES) used to servo the head in response to the servo sectors, thereby defining substantially circular data tracks.
The data tracks may be defined relative to the servo tracks so as to have the same or a different density than the servo tracks (e.g., a higher density than the servo tracks). In either case, at certain radial and/or circumferential locations the servo tracks may be written too close to one another (track squeeze) which decreases the reliability of the resulting data tracks due to adjacent track interference.